Dark Matter in the Universe:Dark Matter in the Universe:

MACHOS, WIMPS andMACHOS, WIMPS andLittle White DwarfsLittle White Dwarfs

Evalyn GatesAdler PlanetariumUniversity of Chicago

What does our Universe look like?What is it made of?How old is it?How will it end?Where do we fit into this picture?

An ancient and still currentquestion:

What is the Universemade of?

Where is the matter?Where is the matter?

!! Most of the matter in the Universe isMost of the matter in the Universe isunseen (dark matter)unseen (dark matter)

!! Most of this dark matter is in the form ofMost of this dark matter is in the form ofexotic new particlesexotic new particles

!! But we havenBut we havent found most of thet found most of theordinary matter either -- where is it?ordinary matter either -- where is it?

Basically, we detect dark matter via its gravitationaleffects on something else.

1st evidence for dark matter: Zwicky (1930s)Coma Cluster of galaxies

Now have evidence on many scales, from the rotationcurves of over 1000 spiral galaxies to gravitational

lensing of clusters of galaxies

http://burro.astr.cwru/JavaLab/RotcurveWeb

Weighing the Universe:

= mass-energy density of Universec = critical density

= /c

< 1 OPEN = 1 FLAT > 1 CLOSED

News Headlines in 2000:

The Universe is FLAT!

Density of matter/energy in Universeis equal to the critical density

http://www.physics.ucsb.edu/~boomerang/

I. Luminous Matter:

Less than 1% of Universeis in visible matter!

Today (z=0) z=2~4 baryons =0.034

stars = 0.0035gas/galaxies = 0.0006 z1100 baryons = 0.04gas/clusters = 0.0025================

II. Ordinary (baryonic) Matter:

Big Bang NucleosynthesisTheory of the formation of the light elements (Deuterium,Helium, Lithium) in the early Universe (about 1 minute afterthe Big Bang) predicts the observed abundances of theseelements if

~5% of the Universe is inordinary matter

Inventory of the UniverseInventory of the Universe

!! Matter in stars & gas (galaxies) < 1%Matter in stars & gas (galaxies) < 1%

!! Ordinary (baryonic) matter 5%Ordinary (baryonic) matter 5%

most of the ordinary matter is most of the ordinary matter is darkdark

III. Total Matter:

Observations of Clusters ofGalaxies indicate

~ 35% of Universe in Matter

Weighing a Cluster of Galaxies:

Searching for Dark Matterin our Galaxy:

Gravitational Microlensing

Milky Way Galaxy

Typical Spiral Galaxy with a disk, central bulge,and large dark halo

Total Mass = 2 trillion times the mass of the Sun

Most (> 90%) of this mass is in the Dark Halo ofthe Galaxy, which extends out over 500,000light-years from the center of the Galaxy --much further than the visible disk

Some (all?) of the dark matter in thegalactic halo could be in the form of

MACHOs

MAssive Compact Halo Objects

Jupiter-sized planetsbrown dwarf starsfaint low-mass starsblack holeswhite dwarf stars

Gravitational Lensing

Einsteins Theory of Relativity => lightbends around massive objects (spacetime iscurved).

A massive object located between Earthand a distant light source can act as agravitational lens.

Paczynski suggested in 1986 that gravitationalmicrolensing could be used to detect MACHOs in thehalo fo our Galaxy.

As a MACHO passes near the line-of-sight to a star inthe Large Magellanic Cloud, the star will appear tobrighten and then return to normal (twinkle!)

How much the star brightens depends on how close theMACHO comes to the line-of-sight.

How long it appears to be brighter depends on how fastthe MACHO is moving and how massive it is.

The LMC is 50 kpc (169,000 light-years) awayfrom Earth.

The rate of lensing events for stars in the LMC (ifthe halo of our Galaxy is made up entirely ofMACHOs) is

0.000001 events/star/year

==> one in a million!

Several experiments have now detectedMACHOs!

The MACHO experiment has reported13-17 eventslasting an average of about 90 days

So what are the MACHOs?

The only hint we have from the microlensingevents comes from the event duration.

The MACHO mass implied by the observedevents is about 1/2 the mass of the Sun

Very faint, low-mass hydrogen-burning starsWhite dwarfs: burnt out cores of stars like theSun

Is the Halo of the Galaxy filled with WhiteDwarfs?

NO!

White dwarfs are remnants of stars like the Sun

~3/4 of the mass of original star is gently blownoff as metal-enriched gas and dust when the starruns out of fuel

~1/4 becomes white dwarf

If even 20% of the Galactic Halo is in the formof White Dwarfs:

Too many metals produced

Too much light produced by original stars

Almost all the baryons in the Universe wereprocessed through the first generation of starsin the Universe

In any case, we now know thatMACHOs make up at most asmall fraction of the darkmatter in the galaxy

Need to look at more exoticdark matter candidates

Sudbury NeutrinoSudbury NeutrinoObservatoryObservatory

1000 ton heavy water detector

SuperKamiokandeSuperKamiokande

50 kton water detectorThreshold ~ 5 MeV

http://superk.physics.sunysb.edu/superk/physics/solar-neutrin

The Sun in Neutrinos (SuperKamiokande Collaboration)

Inventory of the UniverseInventory of the Universe Luminous Matter in stars & gas (galaxies)

Most of the Matter in the Galaxy and theUniverse is likely to be in the form of ColdDark Matter, such as

WIMPS

Weakly Interacting MassiveParticles

Physics beyond the Standard Modelof particle physics

Supersymmetric theories contain extraparticles

LSP= lightest SUSY particle, with amass in the range of20 GeV - TeV may be stable andwould be a good CDM candidate

WIMP SearchesAcceleratorsDirect Detection

Cryogenic detectors which measure theenergy deposited when WIMP collides withnucleus

Indirect DetectionLook for annihilation products (cosmicgamma rays or high energy neutrinos)

Axions: Tunable microwave cavity immersedstrong mag. field

Ordinary matter = 5%luminous < 1%

neutrinos < 1%gas ~ 3%MACHOs ~ 0.5%==============total 5%

WIMPS = 30%

Where is the other 65%???

Inventory of the UniverseInventory of the Universe

Luminous matter in stars & gasLuminous matter in stars & gas(galaxies)

Still Many Questions:Still Many Questions:

What is the Dark Matter?What is the Dark Matter?

What is the Dark Energy?What is the Dark Energy?